1. Field of the Invention
The invention relates to automatic transmissions for automotive vehicles wherein ratio changes between at least one pair of ratios involves engagement or disengagement of one friction element in synchronism with the disengagement or engagement, respectively, of another friction element.
2. Background Art
Multiple-ratio automotive vehicle powertrains include transmissions of the kind shown, for example, in U.S. Pat. Nos. 6,292,731; 5,722,519; 5,553,694; 5,758,302; 6,370,463; and 6,577,939. These are examples of transmissions having gearing with multiple ratios wherein the gear elements of the gearing are controlled by friction clutches or brakes (i.e., torque transfer friction elements) to establish and to disestablish each of several forward driving ratios and one or more reverse ratios.
When the vehicle accelerates from a standing start, the engine delivers power to the traction wheels as the overall transmission ratio progresses from an initial low speed ratio to a high speed ratio with ratio steps between the lowest and the highest ratios. If a ratio change requires engagement of one friction element as the companion friction element is disengaged, the friction elements must engage and disengage in synchronism. Precise synchronization is required to achieve acceptable shift quality. It is necessary, furthermore, for the synchronization to be maintained throughout the life of the transmission, notwithstanding the presence of wear of the torque transfer friction elements and changes in the environmental conditions, such as temperature changes, lubricant viscosity changes, and changes in coefficients of friction for the friction elements.
Errors in synchronization during gear ratio changes cause inertia torque disturbances that have a significant adverse effect on overall shift quality. One of these adverse effects is referred to as a ratio flare condition at the beginning of a ratio change. Another adverse effect is a gear element tie up condition. Either of these adverse effects can cause a perceptible torque disturbance at the torque output shaft for the transmission.
A ratio flare condition occurs when the pressure on the offgoing friction element is too low at the beginning of a ratio change before a torque transfer to the oncoming clutch occurs. To ensure that the oncoming friction element pressure is adequate to initiate a shift and to ensure that the oncoming friction element is filled at the beginning of a shift, the friction element pressure is boosted. If the boost time is insufficient, it is possible to cause a flare condition to occur during torque transfer to the oncoming friction element.
A gear element tie up condition occurs when the friction elements are momentarily applied simultaneously. This can occur if the oncoming friction element boost time is too long.